Investigation of lead borate glasses doped with aluminium oxide as gamma ray shielding materials

Annals of Nuclear Energy 63 (2014) 350–354

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Investigation of lead borate glasses doped with aluminium oxide as gamma ray shielding materials Sandeep Kaur, K.J. Singh ⇑ Department of Physics, Guru Nanak Dev University, Amritsar 143005, India

a r t i c l e

i n f o

Article history: Received 18 February 2013 Received in revised form 30 July 2013 Accepted 2 August 2013

Keywords: Glasses Mass attenuation coefficient DSC Ultrasonic measurements

a b s t r a c t Gamma-ray attenuation coefficients of xPbO(0.90  x)B2O30.10Al2O3 (x = 0.25, 0.30, 0.35, 0.40 and 0.45) glass system have been calculated with WinXCOM computer program developed by National Institute of Standards and Technology. Results have been further used to calculate half value layer and mean free path values. Gamma-ray shielding parameters of glass samples have been compared with standard nuclear radiation shielding concretes. The prepared glass samples have higher values of mass attenuation coefficients and lower values of half value layer as compared to concretes for most of gamma ray energy range. The density, molar volume, X-ray diffraction, UV–visible studies, ultrasonic and DSC investigations have been used to study the structural properties of the glass samples. It has been inferred that increase in the composition of PbO leads to the formation of non bridging oxygens which leads to decrease in the rigidity of the glass samples. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The study of gamma-ray shielding parameters such as mass attenuation coefficient (l/q), half value layer (HVL) and mean free path (MFP) play important role in the research area of radiation physics. Mass attenuation coefficient is the most commonly used parameter to study the interaction of gamma radiations. Concretes are commonly used as shielding materials in nuclear reactors for several types of nuclear radiations including a, b, c and neutrons (Pisarska, 2009; Kaundal et al., 2010; K.J. Singh et al., 2008; D. Singh et al., 2008; Chanthima and Kaewkhao, 2012). Concretes as shielding materials in nuclear reactors suffer from several limitations including the following; (1) Addition of moisture content continuously modify the shielding properties of the concretes. (2) They are opaque to visible light and hence, it is not possible to see through concrete based shield. (3) Crack formation occurs after long exposure to nuclear radiations and aging (Singh et al., 2006; Lee et al., 2007). (4) Loss of water occurs in the concrete based shield due to heat generated at concrete. Heat is generated due to interaction of concrete with nuclear radiations. Heavy metal oxide glasses are one of the possible alternatives of concretes for gamma ray shielding purposes. They are transparent to the visible light and their chemical composition can be varied widely to attenuate several types of the nuclear radiations originating in the nuclear reactors. It has been estimated that oxide glasses can be used as potential candidates as alternatives to ⇑ Corresponding author. Tel.: +91 183 2258809x3165; fax: +91 183 2258820. E-mail address: [email protected] (K.J. Singh). 0306-4549/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2013.08.012

concretes for gamma ray shielding applications. (Manohara et al., 2009). In the light of this situation, authors have selected PbO–B2O3– Al2O3 glass system for the purpose of its study for its application as gamma-ray shielding material. PbO–B2O3–Al2O3 glass system is moisture resistant (Medhat, 2009; Singh et al., 2004). PbO form stable glasses due to its dual nature; one as glass modifier (at low PbO additions) and other as glass former (at higher PbO additions). Pb has higher atomic number in the periodic table which implies lead based glasses may have better gamma-ray shielding properties (Kaundal et al., 2010; Ready et al., 2001). Aluminium oxide is added for increasing the mechanical strength (Andriy et al., 1999). Authors have carried out gamma ray shielding investigations of PbO–B2O3–Al2O3 glasses in terms of mass attenuation coefficient, half value layer and mean free path values. The structural properties are obtained in terms of density, molar volume UV–visible, ultrasonic and DSC measurements for checking the possibility of applicability of the aforesaid system as gamma-ray shielding materials for commercial applications. These glasses are easy to prepare because they have low melting temperatures. 2. Theoretical background Mass attenuation coefficient has been determined theoretically using the WinXCOM computer software developed by National Institute of Standards and Technology (NIST) (Berger and Hubbel, 1987; Gerward et al., 2004). The mass attenuation coefficient is given by;

S. Kaur, K.J. Singh / Annals of Nuclear Energy 63 (2014) 350–354

l=q ¼

X

wi ðl=qÞi ;

ð1Þ

where wi is weight fractions and (l/q)i mass attenuation coefficients of the several elements. Half value layer is calculated with the help of linear attenuation coefficient (l):

HVL ¼ 0:693=l;

ð2Þ

Mean free path has been calculated from the relation described by Tsoulfaniidis (1983). The molar mass Vg is evaluated from;

V g ¼ M=q;

ð3Þ

Here q is the density and the molar mass (M) is given by:

M ¼ xM1 þ ð0:90  xÞM 2 þ 0:10M 3

ð4Þ

Here M1, M2 and M3 are the molar masses of PbO, B2O3 and Al2O3 respectively. 3. Theoretical and experimental details 3.1. Glass preparation Cylindrical shaped glass samples of composition xPbO(0.90  x)B2O30.10Al2O3 in the interval (x = 0.25–0.45) were prepared by using melt quenching technique. For the preparation of glass samples, appropriate amounts of PbO, Al2O3 and H3BO3 (AR grade) were weighed using an electronic balance. The chemicals were mixed in a pestle mortar for half an hour. Porcelain crucible was placed in an electric furnace for about one hour in temperature range from 850 to 950 °C. Dry oxygen was bubbled through melts using quartz tube in order to obtain homogeneity of the glass melt. The melt was poured into a preheated copper mould. The glass samples were then annealed in a separate annealing furnace. All samples were annealed at 270 °C for 12 h. The prepared glass samples were grinded and polished with different grades of silicon carbides and aluminium paper respectively. Densities of these samples was measured by using Archimedes’s principle by using benzene as the immersion liquid (Table 1). 3.2. Gamma-ray shielding properties For the energies varying from 1 keV to 100 GeV, it is expected that WinXCOM computer software can be used as an authentic tool to evaluate the gamma-ray shielding parameters. It has been verified experimentally that WinXCOM computer program gives the results close to experimental results (K.J. Singh et al., 2008; D. Singh et al., 2008; Singh et al., 2006). In the light of this situation, it is speculated that it is possible to obtain authentic data of mass attenuation coefficients of our glass samples and several concretes by WinXCOM computer software in the wide energy range (1 keV to 100 GeV). HVL and MFP parameters are evaluated from mass attenuation coefficients.

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3.3. XRD studies X-ray diffraction studies shows that prepared samples are amorphous. A Philips PW 1710 diffractrometer was used with Cu Ka radiation. The values was recorded at angular range (2h) of 10–70°. Absence of crystallization peak in XRD data shows that prepared samples are amorphous. 3.4. UV–visible investigations UV–visible absorption spectra were taken on polished disc shaped glass samples, in the wavelength range of 200–1100 nm on a Shimadzu double-beam spectrophotometer. The absorption coefficient as a function of wavelength, a(k), was calculated by dividing the measured absorbance by sample thickness. In order to calculate the band gap (Eg), the following relation was used D. Singh et al., 2008);

hx aðxÞ ¼ b½hx  Eg 

n

ð5Þ

where b is constant,  hx is the photon energy, a is the absorption coefficient. and n = 2 for indirect transition. [ hx a(x)]1/2 was plotted as function of  hx for each glass sample. From the linear extrapolation to zero ordinate, the value of Eg was calculated. Urbach energy (DE) was calculated using the following relation (Sharma et al., 2006);

lnðaÞ ¼ C þ hx=DE

ð6Þ

where C is a constant, DE is obtained from the reciprocal of the slope of the graph of logarithm of absorption coefficient, a versus the photon energy. 3.5. Ultrasonic measurements Ulrtasonic measurements were carried out in our laboratory by using Matec equipment (Matec SR-9010 Digitizer and SR-9000 synthesizer). The cylindrical samples having opposite parallel faces were used for ultrasonic measurements. The time of flight of two successive echoes has been measured by using Pulse-Echo mode. All the measurements has been done at 5 MHz by using ultrasonic jelly between the sample and transducer contact. Longitudinal modulus (L) and ultrasonic velocity (VL) are related to each other by the following relation;

L ¼ qðV L Þ2

ð7Þ

3.6. Differential scanning calorimetry (DSC) DSC measurements of the prepared samples were carried out by using Perkin Elmer differential scanning calorimeter in the temperature range of 200–1200 °C with the heating rate of 20 °C/min in nitrogen atmosphere. 10–20 mg of the powdered glass samples were used to perform the DSC measurements. 4. Results and discussion

Table 1 Chemical composition (in mole fractions), density and molar volume of PbO–Al2O3– B2O3 glass samples. Sample no.

Composition (mole fraction) PbO

Al2O3

B2O3

PbBAlG1 PbBAlG2 PbBAlG3 PbBAlG4 PbBAlG5

0.25 0.30 0.35 0.40 0.45

0.10 0.10 0.10 0.10 0.10

0.65 0.60 0.55 0.50 0.45

Density (g/cm3)

Molar volume (cm3/mol)

3.406 3.807 4.152 4.453 4.711

32.66 31.24 30.49 30.15 30.13

4.1. Gamma-ray shielding properties Mass attenuation coefficient, half value layer and mean free path values of prepared samples as function of energy are shown in Figs. 1–3 respectively. For a better radiation shielding material, higher mass attenuation coefficient and low HVL values are required. Mass attenuation coefficient increases with the increase in the lead composition for our prepared glass samples (Fig. 1). Moreover, there is decrease in HVL values with the increase in the content of Pb (Fig. 2). The prepared glass sample which has

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Fig. 1. Variation of mass attenuation coefficient as a function of photon energy (1 keV–100 GeV) in the PbO–B2O3–Al2O3 glass system. Theoretical values at same energies for barite concrete are included for comparison.

Fig. 3. Variation of mean free path as a function of photon energy in the PbO–B2O3– Al2O3 glass system.

attenuation coefficient, half value layer and mean free path values than the aforesaid concretes for most of the gamma-ray energy values. Therefore, it is speculated that prepared glass samples can have advantages over concretes in terms of transparency to visible light, lesser volume requirements and better gamma ray attenuation. It can be concluded from the results that our glass system provides better gamma ray shielding properties than concretes and hence, they can be potential candidates as alternates to concretes for gamma ray shielding applications.

4.2. Structural properties

Fig. 2. Variation of half value layer as a function of photon energy (1 keV–100 GeV) in the PbO–B2O3–Al2O3 glass system. Theoretical values at same energies for ferrite concrete are included for comparison.

higher value of mass attenuation coefficient and lower value of HVL shows better gamma ray shielding properties. In the light of this discussion, it can be estimated that increase in the content of lead oxide improves the gamma-ray shielding properties of the glass samples. Mean free path decreases with the increase in Pb composition (Fig. 3) which justifies our estimation that gamma ray shielding improve with the increase in the content of Pb. Therefore, it is estimated that the glass sample with the highest content of Pb, i.e. 0.45PbO.O.45B2O30.10Al2O3 represents the best glass sample in terms of the gamma-ray shielding properties. Mass attenuation coefficient and HVL values for different concretes used in nuclear reactors are evaluated using WinXCOM computer software for the energies varying from 1 keV to 100 GeV. Barite and ferrite concretes represent the best concretes for mass attenuation coefficient and HVL values (Table 2) and hence, their values have been used in Figs. 1 and 2 respectively for comparison with our glass samples. Our glass samples show better values for mass

X-ray diffraction studies of glass samples have shown the absence of crystalline peaks and presence of a broad halo around 30° which is characteristic of the amorphous nature of the samples. Due to this observation, it can be concluded that our prepared samples are non-crystalline in nature. Glass samples of xPbO(0.90  x)B2O30.10Al2O3 glass system were prepared at the values of x = 0.25, 0.30, 0.35, 0.40 and 0.45. Composition, densities and molar volume values are presented in Table 1. The density values of our glass system increases from 3.406 to 4.711 g/cm3 with the increase in the content of PbO. This can be attributed to higher atomic mass of lead as compared to aluminium and boron. Molar volume decreases from 32.66 to 30.13 cm3/mol with the increase of the mole fraction of lead in lead aluminium borate glass system. The molar volume of the glass samples has been calculated from the ratio of molar mass of glass composition to the density of glass samples. It can be observed that the density increases and molar volume decreases with the concentration of PbO. This feature can be explained as follows. Increase in the density values can be related to the higher atomic weight of Pb (Cevik et al., 2009; George et al., 1999). The molar volume values decrease with the concentration of PbO because PbO plays the dual role. It acts as network modifier at low concentration of PbO and network former at higher concentration of PbO (Kaundal et al., 2010). This leads to decrease in the molar volume values at higher contents of PbO. Band gap and Urbach energies have been evaluated from UV– visible spectra (Table 3). Band gap decreases from 3.52 to 2.76 eV with the increase in mole fraction of PbO, whereas, Urbach energy increases from 0.33 to 0.41 with the rise in content of PbO. The trends in UV–visible light absorption in oxide glasses is due to the excitation of electrons associated with non bridging oxygens

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S. Kaur, K.J. Singh / Annals of Nuclear Energy 63 (2014) 350–354 Table 2 Chemical composition of concretes. Concretes

Ordinary Barite Ferrite

Weight fraction elements H

B

C

O

Na

K

Mg

Al

Si

S

Ca

Fe

Ba

0.0100 0.0083 0.0280

– 0.0115 –

0.0010 – –

0.5291 0.3475 0.4554

0.0160 – –

0.0130 – –

0.0020 0.0022 0.0019

0.0338 0.0044 0.0038

0.3370 0.0148 0.0128

– 0.0997 0.0007

0.0440 0.0834 0.0595

0.0140 0.0047 0.4378

– 0.4237 –

Table 3 Composition, energy band gap (Eg) and Urbach energy (DE) of PbO–B2O3–Al2O3 glass system. Sample no.

Mole fraction (PbO)

Energy band gap Eg (eV)

Urbach energy DE (eV)

PbBAlG1 PbBAlG2 PbBAlG3 PbBAlG4 PbBAlG5

0.25 0.30 0.35 0.40 0.45

3.52 2.89 2.86 2.76 2.76

0.33 0.38 0.40 0.41 0.41

Fig. 5. Variation of ultrasonic velocity and longitudinal modulus with mole fraction of PbO in the PbO–B2O3–Al2O3 glass system.

Fig. 4. Variation of glass transition temperature with mole fraction PbO in the PbO– B2O3–Al2O3 glass system.

(NBOs). Lesser is the concentration of NBOs in the glass network, the higher is the optical energy gap and the lesser are the Urbach energy values in borate glasses and vice-versa. Therefore, UV–visible studies indicate the formation of NBOs at higher contents of PbO. These results indicate that PbO–Al2O3–B2O3 glass system becomes less stable at higher contents of PbO. Fig. 4 shows the variation of glass transition temperature with the mole fraction of PbO of the prepared samples. The values of glass transition temperature decreases from 497 to 422 with the increase in mole fraction of PbO from x = 0.25 to 0.45. Button et al., 1982 had undertaken qualitative analysis between increasing transition temperatures (Tg) and increase in the number of tetrahedral borate units. It has been argued that decrease in Tg and growth of borons with non-bridging oxygens are correlated. Subsequently, Martin and Angell (1984) had quantitatively related glass transition temperature with number of NBOs. The ultrasonic velocity and longitudinal modulus decreases with the increase in mole fraction of PbO (Fig. 5). The formation of non-bridging oxygens (NBOs) at higher mole fraction of PbO may be responsible for the behaviour of the ultrasonic velocity (K.J. Singh et al., 2008; D. Singh et al., 2008). UV–visible data and DSC measurements supports the conclusions of the results of ultrasonic data. When the content of PbO increases, the longitudinal modulus value decreases which

indicates that glasses become less rigid with the increase in the content of lead. Glass samples which have higher value of elastic properties as well as better gamma-ray shielding parameters should have been the ideal choice for its commercial utilization. However, it has been found that addition of PbO content improve the values of gamma-ray shielding parameters but it decreases the values of longitudinal modulus. In the light of this situation, a compromise has to be made between gamma-ray shielding properties and elastic parameters during the choice of the best composition.

5. Conclusions Glass samples of the system PbO–B2O3–Al2O3 can be the potential candidates for gamma ray shielding applications. This glass system improves its gamma-ray shielding properties with the increase in the content of PbO. Results of molar volume, UV–visible studies, DSC and ultrasonic velocity of PbO–Al2O3–B2O3 glass system indicate the formation of more non-bridging oxygens with the increase in the content of PbO.

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